In the following, voltages, which in their voltage range or voltage spread lie above the voltages with which items of low voltage equipment are to be supplied, are designated as “overvoltages”. The term “high voltage circuit” encompasses all circuits, which can provide a voltage, the voltage range or voltage spread of which can exceed the permissible voltage range or voltage spread for external circuit units.
Circuit elements in MOS (Metal Oxide Silicon) technology as well as CMOS (Complementary Metal Oxide Silicon) technology have a range of advantages with regard to high switching speeds at low voltages and currents. A disadvantage of these and other sensitive circuit elements consists in the fact that they can only be operated with limited operating voltages, i.e. the circuit elements are easily damaged or completely destroyed by any overvoltages that occur.
Various process technologies have partially solved this problem by, for example, manufacturing MOS transistors with a thick gate oxide. The thicker a gate oxide of a MOS transistor is, the higher the voltage is, which can be applied to circuit elements of this kind. A disadvantage of MOS transistors with thick gate oxide consists in a reduction of switching speed. The following table, which lists different process technologies, is given as an example of circuit elements with a normal gate oxide thickness compared with circuit elements with an increased gate oxide thickness.
TABLE 1Typical supply voltages for CMOS processesMaximum operatingMaximum operatingvoltage for circuitvoltage for circuitelements withelements withProcess“normal” gate oxideincreased gate oxidetechnoogythicknessthickness0.35 μm3.3 V  5 V0.18 μm1.8 V3.3 V0.13 μm1.2 V2.5 V
The maximum permissible operating voltage is usually 10% more than the nominal value given in the table shown above. When customary circuit elements are operated with a voltage, which exceeds the maximum (e.g. 2 V in a 0.18 μm CMOS process), they will be damaged. The nominal operating voltages for customary circuit elements and for circuit elements with thick gate oxide are designated in the following as VopL and VopH, respectively.
In a disadvantageous manner, many actuators require higher voltages than the maximum permissible operating voltage for CMOS circuit elements. Examples are laser diodes, motors, loudspeakers and chip-to-chip interfaces. If actuators and driver units are constructed on one chip (system-on-chip, SoC), the problems increase. Actuators often work with high voltages (overvoltages), which exceed the voltages listed in Table 1.
Circuit arrangements according to the state of the art are illustrated in FIG. 1. FIG. 1(a) shows an arrangement in which a circuit module 107, which can be operated at low operating voltage and through which a module current 109 flows, is connected to a high voltage circuit 100. The arrow 102 shown in FIG. 1(a) designates a low voltage while the arrow 101 shown in FIG. 1(a) designates an overvoltage. It can therefore clearly be seen that the overvoltage 101 can endanger a correct operation of the circuit module 107, as, under certain operating conditions, in particular when a low current flows through the at least one circuit module 107, a circuit module connection voltage can become higher than a voltage VopL. For example, if the module current is switched off completely, the connecting point between the circuit module 107 and the high voltage circuit 100 can assume the potential of the overvoltage 101 and the MOS circuit elements in the circuit module 107 will certainly be destroyed.
Thus, attempts are made in various ways to protect the sensitive CMOS circuit elements from too high voltages (overvoltages). FIG. 1(b) shows a conventional method for protecting circuit modules 107. In the figure, a low voltage node 110 is shown, to which is connected the high voltage circuit 100, which is connected to the overvoltage 101. Furthermore, one connection of the circuit module 107 is connected to the low voltage node 110 while another connection of the circuit module 107 is connected to ground 108.
The module current 109 flows through the circuit module, as explained with reference to FIG. 1(a). Furthermore, a series circuit consisting of a protection element 103 and a voltage source 106 is connected between the low voltage node 110 and ground 108. A protection element voltage 104 is dropped across the protection element 103 while the voltage source 106 provides a bias voltage 105. This so-called parallel protection device clamps the low voltage node 110 at a voltage, which is not dangerous for the circuit module 107. This is achieved by setting the bias voltage 105 to a value, which corresponds to a difference between the maximum permissible operating voltage of the circuit module 107, i.e. the maximum low voltage 102, and the protection element voltage 104. As an example, a protection element 103 is shown in FIG. 1(b), which is formed by a semiconductor diode. When the voltage across the protection element 103 exceeds a trigger voltage, i.e. a protection element voltage 104, the protection element 103 becomes low resistance and clamps the low voltage 102 to the sum of the bias voltage 105 and the protection element voltage 104.
Further tests have been carried out to overcome the overvoltages 101 present on the high voltage circuits 100. The installation of transistors in the circuit module 107, which are suitable for high voltages of this kind, has been tried. In a disadvantageous manner, the transistors are difficult to obtain and considerably more expensive than conventional transistors. In a CMOS process, they have a thick gate oxide, which has the consequence of a low switching speed and large structural size.
A circuit with parallel protection device according to FIG. 2(b) has the disadvantage that the total current is divided between the high voltage circuit 100 and the circuit module 107 (parallel connection), thus interfering with a total current flow. Furthermore, a modification of the signal, i.e. of the current, which is to be transferred to the high voltage circuit or to the high voltage circuits, occurs in a disadvantageous manner.